The wild-type galectin-9 has a function of suppressing excess immunoreaction or repairing breakdown of immune system such as activation of immunity to cancer, through differentiation or homeostasis of T cells. The wild-type galectin-9 is composed of two Carbohydrate Recognition Domains (CRDs) and a link peptide region linking them. It is suggested that the wild-type galectin-9 that is a recombinant generated using Escherichia coli as a host induces suppression of transfer of cancer and regression of cancer by a direct action on tumor cells (activity of inducing adhesion among tumor cells and apoptosis of tumor cells) and an action via an immune system. Moreover, it is identified that the wild-type galectin-9 does not act on non-activated lymphocytes and induces activated T cells, specifically apoptosis of CD4-positive T cells which cause an excess immunoreaction. Furthermore, it is also identified that the wild-type galectin-9 has potent apoptosis inducibility to synoviocytes involved in such conditions as deformation of joints in rheumatism.
The above-mentioned functions of the wild-type galectin-9 show that the wild-type galectin-9 is useful as therapeutic drugs for various diseases. However, in order to actually distribute the wild-type galectin-9 as a therapeutic drug, there are three problems of protease sensitivity, low solubility, and low yield of recombinant protein. Regarding the problem of protease sensitivity of the wild-type galectin-9 among these problems, the inventors of the present invention have reported the stabilized galectin-9 having a molecular structure more stable to protease (Patent Document 1). However, no effective solution to the problems of low solubility and low yield of recombinant protein has been reported. Therefore, it is strongly required to provide galectin-9 variant superior in solubility and yield in order to commercialize a pharmaceutical utilizing superior functions of the wild-type galectin-9.